Add <PKG>_STRIP_COMPONENTS for packages with non-standard tarballs

Usually package tarballs contain only a directory named
"<pkg-name>-<pkg-version>" which contains the actual source tree. To properly
extract the source files in $(BUILD_DIR), Buildroot passes the
--strip-components=1 option to tar.

Unfortunately a few packages ship in a non-standard way, with no root
directory or with more than one. In Buildroot these packages must be handled by
overriding the <PKG>_DOWNLOAD_CMDS.

As the number of such packages is growing, introduce a <PKG>_STRIP_COMPONENTS
variable for packages to request a number of components to strip different
from 1.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Arnout Vandecappelle <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>